The present invention relates to a method and apparatus for power clamping work pieces at cone shaped surfaces by means of a power clamping vice. More specifically the invention aims at clamping railroad wheels by radially moveable clamping jaws, which engage the cone shaped inner side of a wheel rim. The invention also relates to a power clamping vice for the just mentioned purpose.
Power clamping vices for performing different kinds of clamping methods are known in the art in a multitude of versions. Particularly in connection with vertical turning or milling machines the power clamping vice is frequently constructed in the form of a so-called face plate. This type of structure is particularly used in vertical turning or milling machines preferably designed for machining of railroad wheels. Such power clamping vices are known in many different versions. A particularly good summary or review of the relevant prior art relating to power clamping vices for vertical milling or turning machines particularly for the milling of railroad wheels may be found in the article entitled "Power Operated Clamping Devices for Railroad Wheels and Wheel Rims" published in "TZ for Practical Metal Working" (Kraftbetaetigte Spanneinrichtungen fuer Eisenbahnraeder und Radreifen) 1974, Number 9, starting at page 315.
Especially, if a railroad wheel must be clamped on the radially inwardly facing side of its rim for a milling operation and if this inwardly facing rim side has a conical shape, as is very frequently the case, difficulties have been caused in the clamping due to this conical surface. The railroad wheel has a tendency to slip with the conical inner surface off the clamping teeth of all or of individual clamping jaws during the clamping operation, whereby the wheel slides upwardly even before the clamping teeth have been able to penetrate far enough into the material of the wheel and a further upward sliding cannot be prevented. Due to this slipping and sliding the work piece is not any more supported in a plane and its position relative to reference points is unknown, whereby the automatic control of the milling tools for the subsequent milling operation is substantially prevented.
Efforts have been made to solve this problem by means of a clamping vice as shown in FIG. 10 of the above mentioned article and the respective description. The described construction includes clamping jaws with rigidly arranged teeth.
The clamping jaws are moveable on radially arranged and downwardly inclined guide tracks. The downward inclination is intended to make sure, that the clamping teeth pull the work piece simultaneously downwardly against the face plate or rather the power clamping vice when the teeth penetrate into the material of the work piece. However, practical experience has shown that the success hoped for did not materialize. This lack of success seems to be due to the fact that the amount of upward gliding of the work piece and the amount of downward movement of the clamping teeth carried by the clamping jaws cannot be adjusted or adapted to each other, particularly since the amount of downward movement depends on the inclination of the guide surface. While all clamping teeth are moved downwardly by the same distance which depends on the extend of penetration of the teeth into the work piece, the latter slides upwardly on each tooth by a different amount. This situation is aggravated by the fact that none of the individual sliding-up distances necessarily corresponds in its size to the magnitude of downward movement of a clamping tooth or of the clamping teeth. Thus, the mentioned problem cannot be solved by the prior art power clamping vice.